Transponder devices in the form of Radio Frequency Identification (RFID) tags are well known in the prior art, comprising an integrated circuit with information stored on it and a coil which enables it to be interrogated by a read/write device generally referred to as a reader, for use in a variety of different applications.
Although it is known to provide such RFID tags with their own power source, in many applications the tag is also powered by the radio frequency signal generated by the reader. Such a known system is shown in FIG. 1 where a reader is indicated generally at 10 and a tag at 12. The reader 10 comprises a radio frequency generator 13 and a resonant circuit part 11, in the present example comprising an inductor 14 and a capacitor 15 connected in parallel. The inductor 14 comprises a antenna. The resonant circuit part will have a particular resonant frequency in accordance with the capacitance and inductance of the capacitor 15 and the inductor 14, and the frequency generator 13 is operated to generate a signal at that resonant frequency.
The tag 12 similarly comprises a resonant circuit part generally illustrated at 16, a rectifying circuit part generally indicated at 17 and a memory 18. The resonant circuit part 16 comprises an inductor 19 which again comprises in this example a loop antenna, and a capacitor 20. The resonant circuit part 16 will thus have a resonant frequency set by the inductor 19 and capacitor 20. The resonant frequency of the resonant circuit part 16 is selected to be the same as that of the reader 10. The rectifying part comprises a forward-biased diode 21 and a capacitor 22 and thus effectively acts as a half-ware rectifier.
When the reader 10 is brought sufficiently close to the tag 12, a signal generated by the frequency generator 13 will cause the resonant circuit part 11 to generate a high frequency electromagnetic field. When the resonant circuit part 16 is moved within this field, a current will be caused to flow in the resonant circuit part 16, drawing power from the time varying magnetic field generated by the reader. The rectifying circuit part 17 will then serve to smooth the voltage across the resonant frequency part and provide a DC power supply to the tag's memory 18. The rectifying circuit part 17 is sufficient to supply a sufficiently stable voltage to the memory 18 for the memory to operate.
To transmit data from the tag to the reader, the resonant circuit part is also provided with a switch 23, here comprising a field effect transistor (FET). The FET is connected to the memory by a control line 24. When the switch 23 is closed, it causes an increased current to flow in the tag resonant circuit part 16. This increase in current flow in the tag results in an increased current flow in the reader's resonant circuit part 11 which can be detected as a change in-voltage drop across the reader inductor 14. Thus, by controlling the switch 23, data stored in the memory 18 of the tag 12 can be transmitted to the reader 10.
A problem with such known systems is that although the components of the resonant circuit parts 11, 16 may have the same nominal value, in practice de-tuning of one or both resonant circuit parts can occur, for example because of differences in nominal and actual values of components or from interaction between the antennae 14, 19. The results of such de-tuning can cause undesirable effects. In particular, an amplitude modulated signal can be corrupted into a phase modulated signal with little or no amplitude variation being present. In International Patent Application No. WO 98/20263, a reader is provided which is operable to perform amplitude and phase the demodulation of the returned signal, and also to attempt some tuning of the reader antenna depending on the value of the detected phase between a reference signal and a signal returned from the antenna coil. This solution is however complex and further takes into account of the power supplied to the memory tag.